The invention relates to a compensator comprising a retardation foil at least one optical main axis of which is tilted relative to the normal to a surface of the retardation foil.
A tilted optical main axis is to be understood to mean in this context that the optical main axis makes an angle .alpha., where 0.degree.&lt;.alpha.&lt;90.degree., with the normal to the surface of the retardation foil.
The invention further relates to a liquid-crystal display device comprising such a compensator and a retardation foil as well as to a method of manufacturing a retardation foil.
A retardation foil is to be understood to mean in this context a layer which may or may not be self-supporting and which is made of a birefringent material, or a layer having an optically compensating or delaying effect (an optically anisotropic layer). In the case of birefringence, the refractive index varies as a function of the direction of the vector of the electric field, which direction is associated with a light ray. Birefringent material has only one axis for which applies that a light ray whose vector of the electric field extends along said axis is refracted with an extraordinary refractive index n.sub.c. Said axis is also referred to as the optical main axis of the material. In the case of light rays whose vector of the electric field extends perpendicularly to this axis, the refractive index may be the same in all directions (ordinary refractive index n.sub.o). If, at right angles to this axis, the refractive index varies, then the material is referred to as biaxial material. In this application, "the optical main axis of a layer (foil)" is to be understood to mean the average optical main axis across the thickness of the layer (the foil). Dependent upon the type of material and the structure of the layer, the optical main axis of the material may vary, for example, only in a plane at right angles to the layer. The variation occurs, for example, in the angle which the optical main axis makes with the plane of the layer, so that the effective refractive index varies across the thickness of said layer. Viewed at right angles to the layer, it is also possible, however, that the direction of the optical main axis varies in the plane of the layer. In the former case, complete extinction can be brought about between polarizers crossing each other at 90 degrees; in the latter case, there is always some residual transmission.
The display devices are generally used, for example, in monitors, TV applications and, for example, display devices in motorcars and for measuring instruments. The compensators can also be used in polarizing beam splitters or in laser-optical systems for optical recording.
A compensator of the type mentioned in the opening paragraph is described in PCT application WO 96/06380 (PHN 15.171 or U.S. Pat. No. 8,516,904). In said compensator use is made of an optically anisotropic layer of a cholesterically ordered polymeric material in order to counteract grey-scale inversion in a twisted nematic display device. The polymeric material is ordered in such a manner that a molecular helix can be distinguished, the axis of the helix making an angle with a surface and with the normal to one of the substrates.
In said Patent Application, the possibility of successively arranging a plurality of anisotropic layers having different properties is suggested. A problem which may occur in such a structure is that different anisotropic layers influence each other via the interface. In addition, the manufacture of such an assembly of anisotropic layers is complex because a layer cannot be provided until after the preceding layer has cured.